Conventional electromagnetic actuators have generally utilized the electromagnetic attractive force applied by a magnetic movable member as an electric energy is supplied to an electric coil wound around a magnetic stationary member. Further, another type of conventional electromagnetic actuator known as a latching type electromagnetic actuator uses the magnetomotive force caused by an electric coil as it is energized and the other magnetomotive force caused by a permanent magnet applied to a magnetic movable member in series thereto.
FIG. 4(a) and FIG. 4(b) are schematic structual illustrations for explaining a clapper type electromagnetic actuator which is a typical example of the above described former conventional devices. In the drawings, this type of actuator comprises a magnetic stationary member 3 having magnetic pole faces 3a and 3b, an electric coil 2, a magnetic movable member 4 and a spring 5.
FIG. 4(a) shows one condition in which the coil 2 is not energized. Under this condition, the movable member 4 is maintained in its stable state with keeping some space with respect to the magnetic pole faces 3a and 3b by means of the bias force in the direction represented by the arrow 7 caused by the spring 5. Under this condition, if the coil 2 is supplied with the current of predetermined value, electromagnetic attractive force greater than the bias force generated by the spring 5 is generated between the stationary member 3 and the movable member 4. The movable member 4 is changed into the state as shown in FIG. 4(b) in which the movable member 4 is attracted to the stationary member 3. According to this movement, an actuating linkage, not shown, such as an electric contact, valve rod, or the like is mechanically actuated. This actuator will return to the state shown in FIG. 4(a) when the electric coil 2 is free from the energizing current.
FIG. 5(a) and FIG. 5(b) are schematic structural illustrations explaining a latching type electromagnetic actuator which is the later conventional device described above. This latching type actuator comprises a pair of magnetic stationary members 3, 3 having respective magnetic pole faces 3a, 3b, an electric coil 2, a magnetic movable member 4, a permanent magnet 1 interposed between the stationary members 3, 3, and a spring 5.
In FIG. 5(a), when the coil 2 is not energized, the movable member 4 is kept in its stable state keeping the movable member 4 isolated from the magnetic pole faces 3a and 3b owing to the bias force in the direction represented by the arrow 7 originated by the spring 5. Under this condition, the electric coil 2 is supplied with the current to generate the magnetomotive force having the same polarity as that of the permanent magnet 1. Both magnetomotive forces are duplicated and this duplicated magnetomotive force generates a greater electromagnetic attractive force between the stationary member 3 and the movable member 4 greater than the bias force in the direction represented by the arrow 7 of the spring 5. Thus the movable member 4 is attracted to the stationary member 3 as shown in FIG. 5(b), so that an actuating linkage, not shown, such as an electric contact, valve rod, or the like is actuated.
Under this stable condition shown in FIG. 5(b), even if the coil 2 is free from the energized current, this condition is maintained owing to only the attractive force of the permanent magnet 1.
On the other hand, under the condition shown in FIG. 5(b), when the coil 2 is supplied with the current to generate the magnetomotive force having the counter polarity of the permanent magnet 1, the magnetomotive force of the permanent magnet 1 is cancelled by this counter force. Thus the movable member 4 is returned to its initial stable position shown in FIG. 5(a) by the cancellation and, the bias force originated by the spring 5. According to this manner, this type of actuator can achieve its latching operation.
However, the above described conventional electromagnetic actuators have some problems as follows.
(1) The value of ampere-turns required to energize the gap is too large. Particularly, the latching type actuator requires greater ampere-turns for energizing the coil since the permanent magnet having a great magnetic reluctance is arranged in series in the magnetic circuit which is energized as the coil is supplied with electric current.
(2) In the type in which the current for energizing the coil is continuously supplied to the actuator when the actuating force is generated, the energy consumption is too large in addition to the above condition (1).
(3) The above condition (1) causes the temperature of the electric coil to increase and makes its size larger.
(4) It is necessary to pay attention to treat for residual magnetic flux in case that DC electric magnet is used.
(5) The latching type electromagnetic actuator requires two electric coils for attracting and returning operations or a complicated actuating circuit since the value of ampere-turn required for attracting the movable member is different from that of returning operation.